Help solving a UHV problem (anodized aluminum)

[Ogon Bat]

I recently realized a quite long (4.5 m) tube for ultra high vacuum in alluminium. Unfortunately in the realization a terrible error has been made... The internal surface has been anodized! I know that such treatment horribly enhance the outgassing rate... I read over the net that in principle it is possible to strip the anodized coating using some chemicals... Someone suggest NaOH solution other acid treatment (sulfuric+chromic acid)... but I'm a little bit concerned to destroy the surface, to do worst instead to improve.

Has someone some experience in doing this stuff?

 

[CrysPhys]

I recently realized a quite long (4.5 m) tube for ultra high vacuum in alluminium. Unfortunately in the realization a terrible error has been made... The internal surface has been anodized! I know that such treatment horribly enhance the outgassing rate... I read over the net that in principle it is possible to strip the anodized coating using some chemicals... Someone suggest NaOH solution other acid treatment (sulfuric+chromic acid)... but I'm a little bit concerned to destroy the surface, to do worst instead to improve.

Has someone some experience in doing this stuff?

I worked in UHV a long time ago when 304 stainless steel was the standard material used for UHV vacuum chambers; so I don't have any direct experience with UHV aluminum chambers. But take a look at this: https://www.atlasuhv.com/products/vacuum-chambers/aluminum-vacuum-chambers/.

Assuming the tube was specifically prepared for a UHV chamber (was it?), the anodized coating is designed to passivate the surface and reduce outgassing [that makes a lot of sense to me]. Don't strip off the coating until you check!

 

[Ogon Bat]

Assuming the tube was specifically prepared for a UHV chamber

Thank you for your answer! Unfortunately no, it was a mistake. The anodizing just had to be done externally... the company, absolutely not equipped to prepare UHV components, forgot to protect the inside part. I guess (i.e. I'm sure) that they have done a quite low cost treatment, not specific for UHV. However yes it is a good idea to try first.

 

[berkeman]

However yes it is a good idea to try first.

Will you be able to try it in the next day or two and let us know how it goes? Or will it be a while to put the system together for initial tests?

 

[Ogon Bat]

Will you be able to try it in the next day or two and let us know how it goes? Or will it be a while to put the system together for initial tests?

well the tube I hope will be shipped next week and then I will try with turbo pumps to see how things are going... For now, from the net, it seems a good idea to use a chromic acid/phosphoric acid solution...

 

[CrysPhys]

You mentioned that the tube is quite long (4.5 m). What is the inner diameter?

 

[Dullard]

All out-gassing stops eventually. A combination of vacuum and heat will accelerate that process.

 

[CrysPhys]

All out-gassing stops eventually. A combination of vacuum and heat will accelerate that process.

Well, that depends on how long "eventually" is. That's what distinguishes UHV. You need to be concerned about the outgassing rates and service temperatures (I baked my systems at 300 deg C) of the materials used for the chamber and used for the components within the chamber.

 

[Ogon Bat]

The inner diamater is 6 cm. For alluminium I cannot go over 180°C

 

[CrysPhys]

well the tube I hope will be shipped next week and then I will try with turbo pumps to see how things are going... For now, from the net, it seems a good idea to use a chromic acid/phosphoric acid solution...

I don't quite understand. If the manufacturer messed up, and hasn't shipped yet, why don't you ask them to correct the problem (assuming it is a problem)? Otherwise, assuming you do need to etch away a crappy coating, do you have the facilities to handle etching of such a large (in particular, long) tube?

 

[Jodo]

Sodium hydroxide is commonly used to remove anodized coatings on aluminum.

 

[Ogon Bat]

well now I'm turbo pumping... let's try to see how disastrous the situation is.